1. Technical Field
The present invention relates to a leveling system for use with a vehicle, such as a recreational vehicle.
2. Description of the Related Art
Various systems are known for leveling a vehicle such as a recreational vehicle. These systems are typically hydraulic powered systems that automatically level the vehicles to which there are attached in response to certain operator commands. Some such systems have four spring-biased hydraulic cylinders or “jacks” located one adjacent each corner of the vehicle. (See U.S. Pat. Nos. 4,061,309; 4,165,861; 4,597,584; 4,743,037; and 4,746,133). One problem with four independently operating jacks at each corner of the vehicle is that they tend to twist the vehicle frame during unsynchronized extension and retraction of the jacks.
U.S. Pat. No. 5,176,391, owned by the same assignee as the present application, discloses a vehicle leveling system having three jacks, two of which are located at the rearward end of the vehicle and one of which is located at the forward end of the vehicle. The use of a single front jack reduces twisting of the vehicle frame, however, it makes the system less stable because two corners of the vehicle are left unsupported.
U.S. Pat. No. 5,915,700, also owned by the assignee of the present application and incorporated by reference as though fully set forth herein, teaches a “pseudo four-leg” vehicle leveling system. That system has four jacks with one pair of jacks at one end of the vehicle driven in parallel and the other two jacks being driven independent of each other and the two jacks in parallel. The two jacks in parallel are in fluid communication with each other so that one end of the vehicle can “float” between the common jacks during extension and retraction. In other words, the hydraulic fluid will flow between the parallel jacks to provide more or less pressure in either jack depending on which side of the vehicle exerts more downward force, thereby reducing frame twisting. The stability of the system is also improved because all four corners of the vehicle are supported when the jacks are extended, however, because the hydraulic fluid can flow between the parallel jacks it is less stable than convention four leg systems when the jacks are static.
Accordingly, a vehicle leveling system with improved stability during static operation is needed.